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.Dd March 13, 2022
.Dt MHD 4I
.Os
.Sh NAME
.Nm mhd
.Nd multihost disk control operations
.Sh SYNOPSIS
.In sys/mhd.h
.Sh DESCRIPTION
The
.Nm
.Xr ioctl 2
control access rights of a multihost disk, using
disk reservations on the disk device.
.Pp
The stability level of this interface (see
.Xr attributes 7 )
is evolving.
As a result, the interface is subject to change and you should limit your use of
it.
.Pp
The mhd ioctls fall into two major categories: (1) ioctls for non-shared
multihost disks and (2) ioctls for shared multihost disks.
.Pp
One ioctl,
.Dv MHIOCENFAILFAST ,
is applicable to both non-shared and shared multihost disks.
It is described after the first two categories.
.Pp
All the ioctls require root privilege.
.Pp
For all of the ioctls, the caller should obtain the file descriptor for the
device by calling
.Xr open 2
with the
.Dv O_NDELAY
flag; without the
.Dv O_NDELAY
flag, the open may fail due to another host already having a
conflicting reservation on the device.
Some of the ioctls below permit the caller to forcibly clear a conflicting
reservation held by another host, however, in order to call the ioctl, the
caller must first obtain the open file descriptor.
.Ss "Non-shared multihost disks"
Non-shared multihost disks ioctls consist of
.Dv MHIOCTKOWN ,
.Dv MHIOCRELEASE ,
.Dv MHIOCSTATUS ,
and
.Dv MHIOCQRESERVE .
These ioctl requests control the access rights of non-shared multihost disks.
A non-shared multihost disk is one that supports serialized, mutually exclusive
I/O mastery by the connected hosts.
This is in contrast to the shared-disk model, in which
concurrent access is allowed from more than one host (see below).
.Pp
A non-shared multihost disk can be in one of two states:
.Bl -bullet -width indent
.It
Exclusive access state, where only one connected host has I/O access
.It
Non-exclusive access state, where all connected hosts have I/O access.
An external hardware reset can cause the disk to enter the non-exclusive access
state.
.El
.Pp
Each multihost disk driver views the machine on which it's running as the
.Dq local host ;
each views all other machines as
.Dq remote hosts .
For each I/O or ioctl request, the requesting host is the local host.
.Pp
Note that the non-shared ioctls are designed to work with SCSI-2 disks.
The
SCSI-2 RESERVE/RELEASE command set is the underlying hardware facility in the
device that supports the non-shared ioctls.
.Pp
The function prototypes for the non-shared ioctls are:
.Bd -literal -offset 2n
.Fn ioctl fd MHIOCTKOWN ;
.Fn ioctl fd MHIOCRELEASE ;
.Fn ioctl fd MHIOCSTATUS ;
.Fn ioctl fd MHIOCQRESERVE ;
.Ed
.Bl -tag -width MHIOCQRESERVE
.It Dv MHIOCTKOWN
Forcefully acquires exclusive access rights to the multihost disk for the local
host.
Revokes all access rights to the multihost disk from remote hosts.
Causes the disk to enter the exclusive access state.
.Pp
Implementation Note: Reservations (exclusive access rights) broken via random
resets should be reinstated by the driver upon their detection, for example, in
the automatic probe function described below.
.It Dv MHIOCRELEASE
Relinquishes exclusive access rights to the multihost disk for the local host.
On success, causes the disk to enter the non- exclusive access state.
.It Dv MHIOCSTATUS
Probes a multihost disk to determine whether the local host has access rights
to the disk.
Returns
.Sy 0
if the local host has access to the disk,
.Sy 1
if it doesn't, and
.Sy -1
with
.Va errno
set to
.Er EIO
if the probe failed for some other reason.
.It Dv MHIOCQRESERVE
Issues, simply and only, a SCSI-2 Reserve command.
If the attempt to reserve
fails due to the SCSI error Reservation Conflict (which implies that some other
host has the device reserved), then the ioctl will return
.Sy -1
with
.Va errno
set to
.Er EACCES .
The
.Dv MHIOCQRESERVE
ioctl does NOT issue a bus device
reset or bus reset prior to attempting the SCSI-2 reserve command.
It also
does not take care of re-instating reservations that disappear due to bus
resets or bus device resets; if that behavior is desired, then the caller can
call
.Dv MHIOCTKOWN
after the
.Dv MHIOCQRESERVE
has returned success.
If
the device does not support the SCSI-2 Reserve command, then the ioctl returns
.Er -1
with
.Va errno
set to
.Er ENOTSUP .
The
.Dv MHIOCQRESERVE
ioctl is intended to be used by high-availability or clustering software for a
.Dq quorum
disk, hence, the
.Dq Q
in the name of the ioctl.
.El
.Ss "Shared Multihost Disks"
Shared multihost disks ioctls control access to shared multihost disks.
The ioctls are merely a veneer on the SCSI-3 Persistent Reservation facility.
Therefore, the underlying semantic model is not described in detail here, see
instead the SCSI-3 standard.
The SCSI-3 Persistent Reservations support the
concept of a group of hosts all sharing access to a disk.
.Pp
The function prototypes and descriptions for the shared multihost ioctls are as
follows:
.Bl -tag -width 1n
.It Fn ioctl fd MHIOCGRP_INKEYS "(mhioc_inkeys_t *)k"
.Pp
Issues the SCSI-3 command Persistent Reserve In Read Keys to the device.
On input, the field
.Fa k->li
should be initialized by the caller with
.Fa k->li.listsize
reflecting how big of an array the caller has allocated for the
.Fa k->lilist
field and with
.Ql k->li.listlen\& ==\& 0 .
On return, the field
.Fa k->li.listlen
is updated to indicate the number of
reservation keys the device currently has: if this value is larger than
.Fa k->li.listsize
then that indicates that the caller should have passed a bigger
.Fa k->li.list
array with a bigger
.Fa k->li.listsize .
The number of array elements actually written by the callee into
.Fa k->li.list
is the minimum of
.Fa k->li.listlen
and
.Fa k->li.listsize .
The field
.Fa k->generation
is updated with the generation information returned by the SCSI-3
Read Keys query.
If the device does not support SCSI-3 Persistent Reservations,
then this ioctl returns
.Sy -1
with
.Va errno
set to
.Er ENOTSUP .
.It Fn ioctl fd MHIOCGRP_INRESV "(mhioc_inresvs_t *)r"
.Pp
Issues the SCSI-3 command Persistent Reserve In Read Reservations to the
device.
Remarks similar to
.Dv MHIOCGRP_INKEYS
apply to the array manipulation.
If the device does not support SCSI-3 Persistent Reservations,
then this ioctl returns
.Sy -1
with
.Va errno
set to
.Er ENOTSUP .
.It Fn ioctl fd MHIOCGRP_REGISTER "(mhioc_register_t *)r"
.Pp
Issues the SCSI-3 command Persistent Reserve Out Register.
The fields of structure
.Va r
are all inputs; none of the fields are modified by the ioctl.
The field
.Fa r->aptpl
should be set to true to specify that registrations
and reservations should persist across device power failures, or to false to
specify that registrations and reservations should be cleared upon device power
failure; true is the recommended setting.
The field
.Fa r->oldkey
is the key that the caller believes the device may already have for this host
initiator; if the caller believes that that this host initiator is not already
registered with this device, it should pass the special key of all zeros.
To achieve the effect of unregistering with the device, the caller should pass
its current key for the
.Fa r->oldkey
field and an
.Fa r->newkey
field containing the special key of all zeros.
If the device returns the SCSI error code
Reservation Conflict, this ioctl returns
.Sy -1
with
.Va errno
set to
.Er EACCES .
.It Fn ioctl fd MHIOCGRP_RESERVE "(mhioc_resv_desc_t *)r"
.Pp
Issues the SCSI-3 command Persistent Reserve Out Reserve.
The fields of
structure
.Va r
are all inputs; none of the fields are modified by the ioctl.
If the device returns the SCSI error code Reservation Conflict, this ioctl
returns
.Sy -1
with
.Va errno
set to
.Er EACCES .
.It Fn ioctl fd MHIOCGRP_PREEMPTANDABORT "(mhioc_preemptandabort_t *)r"
.Pp
Issues the SCSI-3 command Persistent Reserve Out Preempt-And-Abort.
The fields
of structure
.Va r
are all inputs; none of the fields are modified by the ioctl.
The key of the victim host is specified by the field
.Fa r->victim_key .
The field
.Fa r->resvdesc
supplies the preempter's key and the reservation that it is requesting as part
of the SCSI-3 Preempt-And-Abort command.
If the device returns the SCSI error code
Reservation Conflict, this ioctl returns
.Sy -1
with
.Va errno
set to
.Er EACCES .
.It Fn ioctl fd MHIOCGRP_PREEMPT "(mhioc_preemptandabort_t *)r"
.Pp
Similar to
.Dv MHIOCGRP_PREEMPTANDABORT ,
but instead issues the SCSI-3 command Persistent Reserve Out Preempt.
(Note: This command is not implemented).
.It Fn ioctl fd MHIOCGRP_CLEAR "(mhioc_resv_key_t *)r"
Issues the SCSI-3 command Persistent Reserve Out Clear.
The input parameter
.Va r
is the reservation key of the caller, which should have been already
registered with the device, by an earlier call to
.Dv MHIOCGRP_REGISTER .
.El
.Pp
For each device, the non-shared ioctls should not be mixed with the Persistent
Reserve Out shared ioctls, and vice-versa,  otherwise, the underlying device is
likely to return errors, because SCSI does not permit SCSI-2 reservations to be
mixed with SCSI-3 reservations on a single device.
It is, however, legitimate
to call the Persistent Reserve In ioctls, because these are query only.
Issuing the
.Dv MHIOCGRP_INKEYS
ioctl is the recommended way for a caller to
determine if the device supports SCSI-3 Persistent Reservations (the ioctl
will return
.Sy -1
with
.Va errno
set to
.Er ENOTSUP
if the device does not).
.Ss "MHIOCENFAILFAST Ioctl"
The
.Dv MHIOCENFAILFAST
ioctl is applicable for both non-shared and shared
disks, and may be used with either the non-shared or shared ioctls.
.Bl -tag -width 1n
.It Fn ioctl fd MHIOENFAILFAST "(unsigned int *)millisecs"
.Pp
Enables or disables the failfast option in the multihost disk driver and
enables or disables automatic probing of a multihost disk, described below.
The argument is an unsigned integer specifying the number of milliseconds to
wait between executions of the automatic probe function.
An argument of zero disables the failfast option and disables automatic probing.
If the
.Dv MHIOCENFAILFAST
ioctl is never called, the effect is defined to be that
both the failfast option and automatic probing are disabled.
.El
.Ss "Automatic Probing"
The
.Dv MHIOCENFAILFAST
ioctl sets up a timeout in the driver to periodically
schedule automatic probes of the disk.
The automatic probe function works in this manner: The driver is scheduled to
probe the multihost disk every n milliseconds, rounded up to the next integral
multiple of the system clock's resolution.
If
.Bl -enum -offset indent
.It
the local host no longer has access rights to the multihost disk, and
.It
access rights were expected to be held by the local host,
.El
.Pp
the driver immediately panics the machine to comply with the failfast model.
.Pp
If the driver makes this discovery outside the timeout function, especially
during a read or write operation, it is imperative that it panic the system
then as well.
.Sh RETURN VALUES
Each request returns
.Sy -1
on failure and sets
.Va errno
to indicate the error.
.Bl -tag -width Er
.It Er EPERM
Caller is not root.
.It Er EACCES
Access rights were denied.
.It Er EIO
The multihost disk or controller was unable to successfully complete the
requested operation.
.It Er EOPNOTSUP
The multihost disk does not support the operation.
For example, it does not support the SCSI-2 Reserve/Release command set, or the
SCSI-3 Persistent Reservation command set.
.El
.Sh STABILITY
Uncommitted
.Sh SEE ALSO
.Xr ioctl 2 ,
.Xr open 2 ,
.Xr attributes 7
